1. Field of the Invention
The invention relates to a piston-operated pipette with a drive unit and a displacement unit which is mounted on the drive unit, displacement unit having a piston drive for at least one piston and being dismountable from the drive unit. In practice, these piston-operated pipettes are used for transfer of liquids using pipette tips.
2. Description of Related Art
The German Patent Application DE-A-198 26 065 discloses a piston-operated pipette which shows a displacement unit as an air-displacement or positive displacement version. The connection on the piston-operated pipette between the drive unit and the displacement unit takes place via coupling apparatus composed of a magnet and a ferromagnetic element as well as means which fit axially into one another. On the one hand, there is a detachable mechanical connection between the piston actuation and the piston rod in the center of the piston-operated pipette. On the other hand, there is a circular ring-shaped detachable mechanical connection between the housings of the drive unit and of the displacement unit. In addition, the displacement unit has an information medium with information about itself and its use. This information is transmitted on the coupling apparatus as galvanic interfaces.
In this case, the two detachable mechanical connections, both of which are made as magnetic connections, are released by manually pulling the drive unit and the displacement unit apart.
These piston-operated pipettes, as so-called multichannel pipettes, have eight or twelve piston-cylinder units, and accordingly, require high actuation forces. The same also occurs in single-channel, large volume pipettes. The detachable mechanical connections or coupling apparatus of the pipette transfer these forces so that their magnetic force must be high. In practice, it has been shown that the magnetic force can be overcome manually only with difficulty. Separation takes place suddenly, is unpleasant, and under certain circumstances can lead to injuries or damage. When the displacement unit is joined to the drive unit by the magnetic force, between the two components injuries can likewise occur.
Another piston-operated pipette is known from International Patent Application Publication WO 02/29419 A2, which corresponds to U.S. Pat. Nos. 7,513,857 and 7,635,326. This piston-operated pipette has a metering head which can be removed by means of a magnetic coupling apparatus. This device has a Cartesian system of axes for moving the metering head. One or more hollow needles into which liquid can be taken and dispensed are located on the metering head. The coupling apparatus is located between the metering head and a retaining device which is located on the system of axes. The metering head and the retaining device have a permanent magnet. To detach the coupling apparatus, the retaining device has an electromagnet which requires its own power supply. For a purely manual piston-operated pipette, this approach is ruled out. Fundamentally, one or more electromagnets are required which increases the weight of the piston-operated pipette considerably.